Films and pigments made from cholesteric liquid crystal polymers (CLCP) are known in the art. Reference is made to U.S. Pat. No. 5,211,877 (Andrejewski et al.); U.S. Pat. No. 5,362,315 (Willer-Rees et al.); and U.S. Pat. No. 6,423,246 (Kasch et al.), which disclose compositions and technology for producing such materials.
Cholesteric liquid crystal polymers show a molecular order in the form of helically arranged molecular stacks. This order is at the origin of a periodic refractive index modulation throughout the liquid crystal material, which in turn results in a selective transmission/reflection of determined wavelengths of light (interference filter effect). The particular situation of the helical molecular arrangement in CLCPs causes the reflected light to be circularly polarized, left-handed or right-handed, depending on the sense of rotation of the molecular helices.
The range of wavelengths reflected by a CLCP is determined by the geometry of its periodic refractive index modulation, i.e. the pitch of the molecular helices, as known to the skilled man. For a given cholesteric liquid crystal precursor material, said pitch depends on a series of selectable factors, among them the temperature, as well as the quantitative presence of solvents and determined chirality-inducing additives; the wavelength of maximum reflection can thus be determined by the chosen manufacturing process. The pitch of the material can finally be frozen by a cross-linking (polymerization) reaction, such that the colour of the resulting cholesteric liquid crystal polymer (CLCP) is no longer depending on external factors.
To achieve this, the monomeric or oligomeric cholesteric liquid crystal material is made to contain reactive groups, such as acrylate and/or methacrylate residues, which can undergo a crosslinking reaction under the influence of UV radiation in the presence of a suitable photoinitiator. Thus, the freezing of the pitch of the suitable oriented CLCP precursor can be simply performed by an exposure to UV-light (UV-curing).
In addition to a determined reflection colour, the cholesteric liquid crystal polymer (CLCP) shows also a more or less pronounced viewing-angle dependent colour variation (‘colour shift’). Films and pigments made of CLCP are for this reason used as security elements on value and identity documents, because the said colour-shifting effect cannot be reproduced by photocopying machines. The reflection band of CLCP materials is relatively narrow and its angle-dependency is given byλrefl.=n*p*cos(α)
wherein λrefl. is the wavelength of maximum reflection; n is the mean refractive index of the material (of the order of 1.5); p is the pitch of the molecular helices; and α is the viewing angle (Eberle et al., Liq. Cryst. 1989, Vol. 5, No 3, 907-916). It is inferred from this formula that increasing the viewing angle causes the reflection wavelength to shift towards shorter wavelengths.
A number of different reflection colours can be realized with a same given CLCP precursor material through appropriately choosing the manufacturing conditions. Further to this, the handiness (left- or right-handed) of the reflection can be chosen as well through the appropriate choice of the chirality inducing additive at the time of manufacturing the material. However, in the field of pigments for security printing, an increase of the number of physically realizable characteristics is perceived as being an advantage, in view of serving a high number of different security document applications.
The number of realizable different optical responses, i.e. ‘colours’ and ‘colour-shifts’, can be substantially increased if different CLCP pigment types, having different optical responses, are combined with each other in a same ink. The production of a security element in such case depends on the availability of two or more different pigments, which are mixed together in the appropriate ratios for serving a determined security document application.
It was perceived that the security level of the CLCP material could be further increased, if the different optical responses could be combined into a same physical pigment, because it is much easier to make up an ink comprising a mixture of a few modular pigments having basic optical responses (i.e. to combine letters of an alphabet), than to manufacture a single pigment which combines optical basic responses into a more complex response (i.e. to find a determined word). Whereas the former can essentially be done in any printer's shop, if the basic pigments are available, the latter can only be performed at the pigment manufacturing facility, and enables therefore a perfect control of the pigment supply chain.
Cholesteric polymeric multilayers, composed from laminated monolayers, have been previously described by Dobrusskin et al. in WO 95/08786. This document discloses a coloured material comprising an aligned chiral liquid crystal polymer (CLCP) lamina of a first kind, and an aligned chiral liquid crystal polymer (CLCP) lamina of a second kind, each lamina being reflective for light in a respective wavelength band when viewed at a given angle, and being solid at room temperature.
To prepare the coloured material of WO 95/08786, the CLCP precursor of a first layer L1 is mixed with a photoinitiator and spread over a flexible carrier sheet S at a first temperature T1, allowing the CLCP precursor to align to form a first colour. The CLCP precursor is then crosslinked by exposing the layer to UV-radiation at said first temperature T1. A second layer L2 is prepared in the same way and spread over the first layer L1 at a second temperature T2, allowing the CLCP precursor to align to form a second colour, and the CLCP precursor is crosslinked by exposing the layer to UV-radiation at said second temperature T2. An embodiment with a first layer shifting from infrared to red, and a second layer shifting from blue to ultra-violet, is disclosed, resulting in a device whose colour shifts from blue to red when going from orthogonal to grazing view.
The double-layer material of WO 95/08786 has, however, the important shortcoming that it cannot be milled down to a pigment. The manufacturing of CLCP pigment comprises the detachment of the polymerized cholesteric layer from the carrier sheet, followed by milling it down to pigment size, suitable for use in inks and coating compositions, using methods known to the skilled man. The double-layer material of WO 95/08786 does not withstand the milling process, thereby decomposing (delaminating) into its individual layers upon detaching it from the carrier sheet, or at latest under the influence of the high energy input in the jet mill, rather than behaving as a single, solid layer throughout the whole process. Using the process and materials disclosed in WO 95/08786, it is therefore not possible to prepare pigments having specific optical properties from cholesteric multi-layers.
In US 2005/266158, liquid crystal bodies such as optical films or reflective polarisers are described. Pigments are not contemplated in the said reference. The said optical films are made to contain up to three different optical layers physically generated from a single coating on a substrate, through subjecting the coating to a sequence of solvent-evaporation- and UV-curing steps. Because of the need for solvent evaporation, the process of US 2005/266158 is however not very suited for the industrial production, due to health, safety and environment concerns.
It was the object of the present invention to overcome the shortcomings of the prior art and to provide pigments having specific, hitherto not available optical properties.